1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction device having two or more of copying, printing, and facsimile functions.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
The image forming apparatuses generally employ either a negative-positive developing system or a positive-positive developing system. While a portion of the surface of the photoconductor exposed to the light beam emitted from the irradiating device is developed in the negative-positive developing system, an unexposed portion of the surface of the photoconductor is developed in the positive-positive developing system. The negative-positive developing system has become common in recent years in digital image forming apparatuses.
In image forming apparatuses employing the negative-positive developing system, an uncharged surface of the photoconductor brought about by a breakdown of the charger or some other malfunction causes an entire portion of the surface of the photoconductor to be developed, resulting in an irregular image throughout which a solid image is formed (hereinafter referred to as a full-page solid image). Similarly, in image forming apparatuses employing the positive-positive developing system, an unexposed surface of the photoconductor caused by a breakdown of the irradiating device or some other malfunction causes an irregular image including the full-page solid image. Continuous image formation in such a state wastes a large amount of both toner and recording sheets. In particular, with facsimile machines, received data is often discarded upon completion of printing of the data for security purposes. Consequently, loss of the facsimile data due to a full-page solid image thus formed causes serious problems because the data cannot be backed up. Therefore, image formation must be immediately stopped upon occurrence of the irregular image including a full-page solid image.
To detect occurrence of a malfunction causing a full-page solid image, one example of a related-art image forming apparatus determines whether or not image data to be written on a surface of a photoconductor includes a full-page solid image. Specifically, occurrence of a malfunction is identified when a density of an image written on the surface of the photoconductor based on the image data indicates that the image includes a full-page solid image even though the image data itself does not include a full-page solid image.
However, because the above-described image forming apparatus identifies the presence of the full-page solid image by calculating the number and size of dots per unit area, extremely precise determination criteria and high accuracy in density detection are required to accurately determine whether the image written on the surface of the photoconductor includes the full-page solid image or merely a high-density image. Further, in a case in which the image forming apparatus includes multiple photoconductors, a density detector must be provided to each of the photoconductors to detect a toner density of each image formed on surfaces of the photoconductors, causing cost increase.